The present invention relates to a fuel injection control system including an apparatus for driving a fuel injection valve in such a manner that the air-to-fuel ratio of the engine with which the system is used is maintained constant.
FIG. 1 is a block diagram showing an essential portion of a fuel injection control system for an internal combustion engine to which one embodiment of the present invention can be applied. An arithmetic control system 3 receives, as operating parameters, both (1) the output of an engine speed detecting device 7, which generates a pulse each time the crankshaft (not shown) of the engine rotates through a predetermined angle, specifically, for each intake stroke of the engine, and (2) the output of an intake air flow rate detecting device, specifically, a pressure sensor 2 disposed in an intake manifold of the engine downstream of a throttle valve 5. In response to these parameters, the arithmetic control system calculates an approximate driving time of a fuel injection valve 4, disposed downstream of an air cleaner 1, which injects fuel into a cylinder 6 in synchronization with the rotation of the engine.
In the system of FIG. 1, fuel pressurized by a fuel pump 9 is supplied from a fuel tank 10 through a fuel pressure regulator 8 by way of a fuel line 13 to the fuel injection valve 4. The fuel pressure regulator 8 is connected by a line 14 to the intake manifold at a point adjacent the fuel injection valve 4 so that the pressure at the injecting position of the fuel injection valve 4 may be used as the operating pressure of the fuel pressure regulator 8. Pressurized excess fuel is returned to the fuel tank 10 via a fuel line 12. With the described arrangement, the pressures upstream and downstream of the injection valve 4 are held at predetermined levels.
In FIG. 2, which shows an example of a conventional fuel injection control system, reference numeral 20 indicates a sawtooth wave generating circuit which is triggered by the output of the engine speed detecting device 7. The output of the sawtooth wave generating circuit 20 is connected to one input terminal of a comparator 30, the other input terminal of which is connected to the output of the pressure sensor 2 which generates a voltage which is linearly proportional to the absolute pressure in the intake manifold downstream of the throttle valve 5. In this arrangement, the comparator 30 outputs a signal which drives (opens) the fuel injection valve 4 when the output of the sawtooth wave generating circuit 20 is lower than the output of the pressure sensor 2, with the driving of the fuel injection valve 4 commencing from the time the sawtooth wave generating circuit is triggered by the output of the engine speed detecting device 7. The output of the comparator 30 is applied to the fuel injection valve 4 through a driver 40.
This system is constructed and operates upon the assumption that a linear relationship exists among the intake air flow rate, the absolute pressure in the intake manifold, the output voltage of the pressure sensor and the effective driving time of the injection valve during one intake stroke of the engine, and also that a linear relationship exists between the effective driving time of the fuel injection valve and the amount of fuel injected. The amount of fuel injected from the fuel injection valve in one operation is dependent upon the effective area of the valve, the open time of the valve and the pressure of the fuel supplied thereto. Of these parameters, the effective area of the valve is assumed to be an invariable. Therefore, if the fuel pressure is held constant, assumedly a linear relationship exists between the effective driving time of the fuel injection valve and the amount of fuel injected.
The actual relationship, however, between the driving time of the fuel injection valve and the fuel discharge amount in this system is as indicated by a solid curve (a) in FIG. 3. From FIG. 3, it may be seen that a linear relationship, indicated by a broken line (b), is present only for a driving times longer than a minimum time t.sub.0. The nonlinearities at driving times shorter than t.sub.0 can be attributed to the fact that the effective area of the valve is in a transient state during transitions of the valve between open and closed states. At the drive time t.sub.0, the actual effective area of the valve reaches the theoretical fixed effective area. As shown in the graph of FIG. 4 which plots the fuel flow rate of the valve versus times, the injection valve 4 begins to open, following application of the driving signal thereto at t=0, at a time t.sub.2. After gradually opening to the theoretical fixed area between t.sub.2 and t.sub.3 and remaining fully open until the end of the calculated driving time at t.sub.4, the valve gradually closes until it is completely closed at t.sub.5.
The areas A, B and C under the curve in FIG. 4 represent the total amount of fuel injected by the valve in the corresponding time periods. It is the existence of the areas A and C for the periods from t.sub.2 to t.sub.3 and from t.sub.4 to t.sub.5 which make the actual relationship between the driving time of the valve and the amount of fuel injected nonlinear. The presence of the areas A and C is unaffected by changing the theoretical fixed effective area of the valve, the fuel pressure, or the fuel line size. In order to reduce the areas A and C to zero to reduce the time t.sub.0 to zero, the fuel injection valve would have to be opened and closed at an infinite speed, which is clearly impossible for a valve body having a finite inertia. Moreover, even a significant reduction of t.sub.0 would require a very expensive injection valve and driver.
Moreover, a practical fuel injection valve must have a minimum injection (open) period determined by the maximum rotational speed of the engine. Specifically, the valve should be able to open and close about five times within a period defined by t.sub.1 -t.sub.0 in FIG. 3. However, it is difficult as a practical matter to construct a fuel injection valve which meets this criteria. To compensate, prior art fuel injection systems used a plurality of injection valves or they operated the injection valve only outside of the non-linear region. This was accompanied by a difficulty that the air-to-fuel ratio could not be precisely controlled.
It is thus the primary object of the present invention to eliminate the aforementioned defects of prior art fuel injection systems.